The invention pertains to the art of switching circuits and is applicable to full-bridge DC/DC converters which have minimal drain voltage on the switches during a turn on. Switching circuits having the above attribute being commonly known as resonant or soft switching full-bridge converters.
It will be appreciated, however, that the invention has broader applications and may be advantageously employed in other environments and applications that wish to achieve minimization of energy loss.
Resonant or soft switching circuits are often employed due to their characteristics of reduction of power losses which allow for high efficiency switching and reduction of electromagnetic interference (EMI) generation. It is known, however, that even with such circuits switching losses due to the use of rectifier diodes are not substantially diminished. Further, the combination of the reverse-recovery process of the rectifier diodes with the leakage inductance of the transformer causes voltage overshoot and ringing that will lead to excessive dynamic losses or the failure of the rectifier diodes. The severity of this problem will tend to increase as the rectifier diode breakdown-voltage rating increases, since the diode reverse-recovery time increases with increasing voltage rating.
It is well known that diode recovery losses can be reduced by the insertion of an inductor in series with the power transformer primary. Another advantage of this inductor is to reduce the converter load value at which resonant transition occurs.
Various solutions to voltage overshoot have been proposed including the use of clamping circuits, and RC snubbers connected across the rectifier diodes. U.S. Pat. No. 5,198,969 is illustrative of such attempted solution. This patent includes clamp diodes connected to provide a path to the input supply to reduce switching losses and voltage stresses of rectifier diodes. This circuit acts to block the transmission of excessive energy from the primary side of the circuit to the secondary rectifier circuit through the use of the clamping diodes.
It has been found, however, that the above proposed solutions are relatively inefficient and/or complex, and while reducing voltage overshoot, some also may dissipate or otherwise remove energy from the circuit thereby reducing the efficiency of the circuit.
Therefore, it has been deemed desirable to find a circuit which will reduce the voltage overshoot in a resonant bridge converter, which will also recover energy otherwise lost in the process of eliminating the voltage overshoot.